In a system according to a first related art, in order to perform induction heating on a workpiece according to the shape of the workpiece or a portion of the workpiece to be heated, or according to disposition of a coil with respect to the workpiece, a workpiece supporting manner, or the like, different types of multiple induction heating apparatuses are disposed. For example, when induction heating is performed, the penetration depth of a magnetic flux, which is generated from a heating coil, from the outer surface of a workpiece into the workpiece depends on a frequency, according to the thickness of a heat treatment layer, a frequency is selected. In order to thicken the heat treatment layer, a low frequency is used, and in order to make the heat treatment layer shallow, a high frequency is used. To this end, there is a system configured by disposing power sources having different output frequencies and connecting the power sources to induction heating apparatuses through switches, respectively, to perform induction heating on workpieces by the different frequencies (see, e.g., JP60-249288A).
Further, recently, induction heating has been performed using a plurality of frequencies, not one frequency. For example, induction heating has been performed by superimposing a low frequency and a high frequency at the same time.
However, if a power supply system for outputting electric power of a plurality of frequencies is disposed with respect to one induction heating apparatus, equipment becomes large in scale, and an induction heating system becomes expensive. Also, in the system disclosed in JP60-249288A, it is impossible to attach heating coils different in shape or size in the induction heating apparatuses, and freely set a time chart of power supply to each heating coil. Further, if it is assumed to make the thickness of the heat treatment layer different for each workpiece or to perform various heat treatments such as quenching and tempering in the individual induction heating apparatuses, since load impedances including workpieces are different with respect to the power supply system, it is necessary to provide a large-scale power supply system or matching circuit, and thus the entire induction heating system becomes large-scale.
In a system according to a second related art, one power supply apparatus is used to supply electric power to a plurality of induction heating apparatuses. This system includes, for example, a high-frequency power source, a current transformer having the primary side connected to the high-frequency power source, and a plurality of induction heating coils connected in parallel to the secondary side of the current transformer (see, e.g., JP2009-158394A). In this system, a voltage detecting sensor is provided on the secondary side of the current transformer, and a current detecting sensor is provided at a position adjacent to the induction heating coils. On the basis of the value of a voltage which the voltage detecting sensor detects, and the value of a current which the current detecting sensor detects, the magnitude of electric power being supplied to the induction heating coils is monitored.
However, when the power supply apparatus outputs electric power by a time-division multiplexing method or a superimposing method, it is impossible to monitor the output situation. Also, in a case where a plurality of power supply apparatuses supplies electric power according to a supply condition requested by each induction heating apparatus, there is no method of confirming whether electric power is being supplied according to the supply condition.